1. Field of the Invention
This invention relates to an improved process and apparatus for the distillation of aqueous liquids and is particularly useful in the distillation of sea water to produce fresh water.
2. Description of the Prior Art
Distillation is a process for vaporizing a liquid and then condensing the vapor. It is useful in separating volatile portions of a mixture from non-volatile or less volatile components.
A practical distillation device must effect this separation at a low cost in both energy and in capital. Only when both of these cost elements are low is a distillation device or process likely to be useful. Energy efficiency is commonly measured as a "performance ratio", which is the amount of latent heat recovered divided by the amount of heat applied to the system. A high performance ratio in a device implies low energy cost. Typical performance ratios for commercial distillation plants are in the range of six to twelve. Capital cost depends on the cost of component materials, the amount of material needed, and the complexity of the system. Currently available distillation systems are expensive, because they require exotic alloys and are mechanically complex.
Attempts have been made to construct distillation apparatus using porous materials through which the liquid vapor can diffuse, and U.S. Pat. No. 3,340,186 to Weyl is an example of previous efforts using microporous, hydrophobic PTFE membrane. For such "membrane distillation" apparatus, capital cost can be related to the amount of distillate produced per unit area of membrane per unit time. In what follows, this will be referred to as the "productivity" of the device or process. The more productive a device, the lower its capital cost is likely to be per unit of distillate produced.
It is difficult to devise a distillation process which is both energy efficient and productive. In any such process, increasing the productivity by increasing the temperature difference between the warm evaporating salt water and the cooler salt water in the condensor will result in a decreased performance ratio. The objective, therefore, is to decrease this temperature difference while maintaining the same productivity. This may be achieved by decreasing the "vapor gap distance", that is, the distance which the vapor must travel from the point of evaporation to the point of condensation; decreasing the thickness of the distillate layer; improving the mixing of the salt water within the channels; and/or using a more thermally conductive material for the condensor.
Attempts have also been made to construct "sandwich" or multi-effect devices as a means of recovering the latent heat of the condensate, but to a large extent these attempts have not resulted in a practical distillation device. Mixing within the salt water channels can be poor, because attainment of temperature differences across the membranes depends on the flow of the salt water being kept relatively slow. The resultant poor mixing results in large temperature drops across the salt water layers, which substract from the useful temperature drops across the membranes. Moreover, poor mixing results in stagnant concentrated salt water layers at the interface of the membrane and the warm salt water. These lower the vapor pressure of the interfacial salt water and decrease productivity. They may also become supersaturated and "wet out" the membrane, thereby contaminating the distillate with feed water. See German Offenlegungsschrift No. 30 05 192 to Cheng.
Another major deficiency of previous multi-effect distillation devices is that no means is provided for removal of the distillate so as to maintain a minimal thickness of the distillate layers. Because temperature drops across the distillate layers subtract from the temperature differences across the membranes, they decrease either the productivity or the performance ratio. U.S. Pat. No. 3,563,860 to Henderyckx discloses a distillation apparatus using permeable cellulose acetate membranes with an air gap being maintained between the membrane and a condensor wall. The necessity for a sizable air gap can limit the productivity of such a device, but without the air gap, distillate touching the cellulose acetate membrane would tend to pass through it back into the salt water by osmosis, and so the air gap must be large enough to prevent this. Because such a distillation device would have to be very long in order to recover a significant amount of latent heat, removal of the distillate would be a problem.
Possibly in appreciation of this potential problem, Henderyckx suggests forcing the distillate out by air pressure or by gravity. Considerable air pressure would be required if the gaps were thin, and the flow of air would be expected to blow out a good deal of warm water vapor as well as distillate and thus lower the productivity. Use of gravity would require large vertical spaces increasing the size and thus capital cost of the apparatus.
Neither of the above patents cites experimental evidence of a device which demonstrates either the productivity or the combined productivity and performance ratio necessary for low-cost distillation. Henderyckx gives a theoretical example of a distillation device of his design achieving a performance ratio of 48 by means of the extremely small driving temperature difference of 1.degree. C. However, the productivity under these conditions is negligible and the device is really operating as a well insulated heat exchanger. Henderyckx does not give productivity figures in his calculations. Work with diffusion stills, however, has demonstrated that at a 50.degree. C. average working temperature, the 7 mm vapor gap suggested by Henderyckx will allow only 0.03 gal/ft.sup.2 -day-torr of water vapor to diffuse through, or a total of 0.03.times.2.9 torr=0.087 gal/ft.sup.2 -day where the feed is seawater. This calculation does not include temperature drops across stagnant brine layers, across the distillate layer, across the cellulose acetate membrane, and across the condensor. Even if these other deleterious factors are not considered, the productivity is only one-twentieth that of a practical device. To be useful, a desalination process must demonstrate both good productivity and good energy efficiency.